Implement SAND-WP-0005: extension SDK and ext.vm-packer

Add SandboxExtension base class, extension SDK docs, vm-packer attach mode
for build-machines VMs, profile.vm-haskell-build, SSH port support, tests,
and migration docs.

tests/test_extension_registry.py

@@ -0,0 +1,15 @@
+"""Extension registry loads vm-packer."""
+
+from sandboxer.extensions.registry import load_all_extensions, load_extension
+
+
+def test_load_vm_packer_extension() -> None:
+    ext = load_extension("ext.vm-packer")
+    assert ext.id == "ext.vm-packer"
+    assert "vm_packer" in ext.handler
+
+
+def test_load_all_includes_vm_packer() -> None:
+    extensions = load_all_extensions()
+    assert "ext.compose-ssh" in extensions
+    assert "ext.vm-packer" in extensions